Conventional flutter is an unstable aeroelastic coupling between inertial elastic forces and unstable linear aerodynamic forces, which is characterized by the appearance of divergent oscillations, susceptible of rapidly growing use to destruction of the aircraft.
Limit cycle oscillation, when it is dampened, stabilizes at a certain constant amplitude adapted to affect more or less strongly the comfort of the passengers and the crew, or even the maneuverability of the aircraft, and to give rise to more or less damaging consequences for the aircraft according to the amplitudes reached and the frequencies.
Limit cycle oscillations result from a coupling of the same nature as that of conventional flutter and belong to a vibratory regime having in common with this latter the fact that it arises at a fixed frequency, generally less than 20 Hz, and more particularly in aircraft of the freight type, less than 5 Hz, this frequency depending on damping of the coupled vibration modes.
The phenomenon of limit cycle oscillation can appear in fields of flight in which there is the risk of flutter, but it can also appear in fields of flight where there is no risk of flutter. It can be explained by the existence, under conditions of transonic flight, of aerodynamic non-linearity. The phenomenon of limit cycle oscillation, under certain conditions of loading, of Mach and of speed, can appear spontaneously (without particular impetus), or as a result of turbulence or movements imposed by controls, or be caused for experimental purposes by deflection in crenelations of the ailerons.
The resulting movement, when it is established, translates into complex movements of pitching (not necessarily in phase) of the two outside motors of the four-motors, by lateral bending of these same external motors, by bending and twisting of the wings and by vertical bending of the fuselage.
It has thus been possible to determine that vertical accelerations of the front part of the fuselage reaching .+-.0.2 g, give rise to quite uncomfortable vibrations.
To overcome conventional flutter, recourse has already been had in the past to the addition of identical masses on each wing of a plane; it is thus well known that the installation of masses placed symmetrically in the right and left wings is effective to push back the speed limit above which appear vibrations in the structure. However, the disappearance of the divergent oscillations resulting from flutter is paid for by an increase in carried weight which is prohibitive, contrary to the requirements of greater and greater increase of the ratio of useful load to empty weight of the airplane.